TF broken and not correct in Hector Pose Estimation when fusing IMU and Pressure Sensor

edit

Im doing some underwater UUV Gazebo simulation and would like to use the

hector_pose_estimation

package to fuse IMU and Fluid pressure sensors input for pose estimation. In the gazebo launch file I add this tf transformation <node pkg="tf" type="static_transform_publisher" name="world_to_nav" args="0 0 0 0 0 0 /world /nav 100" /> . Im not sure is need it and is the correct one, so the tf looks like this

when using the following hector_pose_estimation launch file

<?xml version="1.0"?>
<launch>
  <node pkg="hector_pose_estimation" type="pose_estimation" name="hector_pose_estimation">
    <remap from="raw_imu" to="/rexrov2/imu" />
    <remap from="pressure_height" to="/pressure_height" />
  </node>

   <node pkg="tf" type="static_transform_publisher" name="nav_to_base_footprint" args="0 0 0 0 0 0 /nav /rexrov2/base_footprint 100" />
</launch>

The TF is following

The both TF from gazebo and hector_pose_estimation are not correct. Any help how to get the correct frames to make it work? And when using the above launch the hector_pose_estimation /pose is like these

header: 
  seq: 329
  stamp: 
    secs: 781
    nsecs: 118000000
  frame_id: "nav"
pose: 
  position: 
    x: 0.0
    y: 0.0
    z: -1.64102132948e-09
  orientation: 
    x: -8.80621330591e-06
    y: 0.000294167493465
    z: 3.71623915803e-14
    w: 0.999999956694

Which is not correct as I have the true pose which if following

header: 
  seq: 15460
  stamp: 
    secs: 781
    nsecs: 608000000
  frame_id: "world"
child_frame_id: "rexrov2/base_link"
pose: 
  pose: 
    position: 
      x: 0.083413690347
      y: 0.133535986662
      z: -0.77756440376
    orientation: 
      x: -9.34768900238e-06
      y: 0.000294151069482
      z: 0.00184072884918
      w: 0.999998262551

In the hector /pose the frame_id: "nav" and no child_frame. In the real pose is frame_id: "world" and child_frame_id: "rexrov2/base_link" (rexrov2 is the name of the robot) . So any help how to fix it?

Thanks